The primary objective of this study is to evaluate the safety of MK-639 administered in combination with Zidovudine (ZVD) and 3TC. The second objectives are: (1) to evaluate the antiviral activity of the combination of ZVD and 3TC and MK-639 in patients previously treated with ZVD; (2) to assess the development of resistance to all three agents; (3) to assess plasma concentration of MK-639 when given concurrently with ZVD and 3TC.